As an III group nitride semiconductor device has a wide band gap, a high dielectric breakdown field intensity and a high electron saturation drift velocity as well as other characteristics, it is suitable for being used to produce an electronic device which has capacity of withstanding a high temperature and a high speed conversion as well as has a high power. In a nitride field effect transistor, a large amount of charges are generated in a channel layer by piezoelectric polarization and spontaneous polarization. Since two-dimensional electron gas is formed by ionization of a donor-type surface state of a nitride surface, a current density of the nitride transistor is extremely sensitive to the surface state, the presence of the surface state can easily cause a current collapse effect.
GaN-based field effect transistors are usually classified into two types according to gate structure, that is, Schottky gate field effect transistors and insulated gate field effect transistors. For Schottky gate field effect transistors, it is easy to form a gate electrode of a Schottky contact and to control surfaces, which is ideal for a RF device. However, because Schottky gate metal and a nitride semiconductor layer are not separated by a dielectric layer, a leakage current of the gate electrode is relatively high and is increased rapidly with the increase of a reverse bias. In addition, due to restrictions for forward conduction of a Schottky contact, a bias on the gate electrode cannot exceed 2V principally, otherwise the gate electrode will lose control of the channel. Therefore, due to lack of a gate electrode having insulated dielectric, a Schottky gate field effect transistor has some issues such as a high gate leakage current and a low gate bearable voltage. For an insulated gate field effect transistor, usually a dielectric layer containing, e.g., one of silicon dioxide, aluminum oxide, hafnium oxide, silicon nitride and silicon oxynitride is disposed below the gate metal, so that the gate leakage current is relatively low, which is suitable for power devices. Therefore, due to the insulated dielectric, the insulated gate field effect transistor has a low gate leakage current and a high gate bearable voltage. But in an insulated gate field effect transistor manufactured in this way, as shown in FIG. 1, there is an interface state having a high density between the dielectric layer and the nitride semiconductor, and thus a serious current collapse effect may be caused, which is a big issue to be resolved. Furthermore, when there is a relatively high interface state between the insulating dielectric layer and the nitride semiconductor layer (such as between Al2O3 and AlGaN), under a forward bias, charging and discharging of the interface state at edges of AlGaN conduction band will lead to a lag effect of a C—V curve of the device, namely as shown in FIG. 2, a forward curve and a reverse curve do not overlap in a large extent. Therefore, it is very difficult to find a field effect transistor structure and a manufacturing method thereof which can result in a relatively low interface state for the insulated gate field effect transistor so as to minimize the current collapse effect.
Therefore, in view of the above-mentioned technical problems, it is necessary to provide an III group nitride semiconductor device and a manufacturing method thereof.